This invention relates to a "defect-free" bipolar process, and also to a CMOS self-aligned process which is described herein and is the subject of a commonly assigned, co-pending patent application entitled SELF-ALIGNED CMOS PROCESS.
Any degree of damage or defect formation introduced into silicon during processing degrades yield and performance. This degradation is especially pronounced in sub-micron and deep sub-micron structures. For example, when such damage occurs in source/drain regions of CMOS devices, subtle effects such as increased leakage currents, localized avalanching, and increased device failure rates can result.
Direct implantation of dopants into designated regions of the silicon for CMOS source/drain or bipolar emitter/collector formation introduces damage. This type of damage is often very difficult to remove. Moreover, permanent damage results from lattice displacements which are generated by the electronic energy loss of the implanted dopant species along its path in the silicon.
Deposition of germanium or polycrystalline silicon-germanium (Si.sub.1-x Ge.sub.x) alloys may be made selective with respect to silicon and SiO.sub.2 surfaces. It has been demonstrated experimentally that deposition of germanium or silicon-germanium alloys onto SiO.sub.2 can be totally suppressed. This feature permits either germanium or alloys of silicon and germanium to be used in self-aligned processes both as masks and as dopant sources. This is particularly relevant when germanium is used in the silicon substrate to control diffusion of n-type dopant species. Implantation of n-type dopants into regions of silicon that have very high concentrations of germanium, by theory and experiment, lead to damage freeze-in. That is, damage is permanent because of the attractive interactions between the n-type dopant and the germanium, as well as the interstitial silicon in the damaged portions of the silicon and the germanium. Therefore, although germanium can be used to control diffusion of n-type dopants, permanent damage may be introduced if the n-type dopant is implanted in high doses. One alternative is to deliver the n-type dopant to the substrate by another process which shifts any damage to materials that will ultimately be removed from the structure. This is a general approach, as will be illustrated below.
Selectivity is not only confined to deposition processes but also can be developed with etching processes. Conventional selectivity between SiO.sub.2 and polysilicon extends to poly-SiGe films as well as germanium. Moreover, there are dry etching processes that can be made selective with respect to germanium versus silicon.
In light of the foregoing, it is evident that there is a need for processes that will suppress the defect generation due to the implantation of impurities during semiconductor fabrication.